New research paper on Browning of the Baltic Sea: why decrease in water transparency matters at the land-sea interface.
Research co-funded by LandSeaLot reveals an increase in coloured dissolved organic matter (CDOM) in the Gulf of Bothnia, the eastern Gulf of Finland, and the eastern Gulf of Riga. This study builds on previous research that combined in situ measurements with satellite observations to track changes in Baltic Sea water transparency since the early 1900s. The researchers extended this record to 2025 and evaluated trends in satellite-derived water transparency between 1997 and 2025.
Where does coloured dissolved organic matter (CDOM) come from?
CDOM mainly originates from decomposing plant material and strongly absorbs ultraviolet and blue light. The increase in CDOM observed in the study is likely linked to climate change altering rainfall, river flow,and river runoff. Together these factors cause more organic material from soils and vegetation to wash into the Baltic Sea.
What is the impact of CDOM?
As CDOM concentrations increase, the water becomes browner and darker. This browning reduces the amount of light available underwater, potentially affecting primary production, the solar bleaching of organic material, phytoplankton physiology, and the behaviour of zooplankton and fish.
What do the findings suggest?
The study suggests that the Gulf of Bothnia and the eastern Gulf of Finland are experiencing a major environmental change associated with the browning and darkening of the water.
What can we do about it?
The study calls for stronger monitoring of this phenomenon and its impacts. This will require greater investment in open-sea in situ observations and their integration with existing river, coastal and satellite observations. This dovetails with LandSeaLot’s mission to integrate and enhance coastal observation methods to solve common challenges.
LandSeaLot in the News
This research was recently featured in the news, when LandSeaLot researcher Jukka Seppälä from the Finnish Environment Institute (Syke) was interviewed by Yle, the Finnish national broadcaster, about the browning of the Baltic sea causes and potential impacts.
Thank you to the authors and institutions involved in this research:
Mati Kahru, Bronwyn Cahill, Samu Elovaara, Zhongping Lee, Gregor Rehder, Jukka Seppälä, Pasi Ylöstalo and Oliver Zielinski
Scripps Institution of Oceanography, University of California San Diego, Leibniz Institute for Baltic Sea Research Warnemünde (IOW), Finnish Environment Institute Syke, College of Ocean and Earth Sciences and University of Rostock
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